1. Field of the Invention
The invention relates to a sintered composite machine part suited to a machine part required to have an airtightness and a locally high sliding property such as a cylinder block used in motion conversion between flowing fluid and rotary mechanism and its manufacturing method, and a cylinder block that can be manufactured by applying the same.
2. Related Art
A cylinder block is an element for composing a skew piston pump or skew piston motor, and is a rotating member having a plurality of cylinder holes with a bottom which are formed in parallel to a rotary shaft at equal intervals along a circumference concentric with the rotary shaft. By using the cylinder block in combination with a piston fitted into each cylinder hole, and a skew plate configured to rotate in connection with the reciprocal motion of the piston, motion can be converted between the flow of the fluid and the rotary motion of the shaft. Then, if fluid (gas) is supplied/discharged in the cylinder holes from outside by way of through-holes on the cylinder hole bottom, the skew plate is rotated by reciprocal motion of the piston, and the system acts as a skew piston pump for converting the ejection pressure of external gas into rotary motion of skew plate. To the contrary, if rotating the skew plate by using an external power source, the external fluid (liquid, gas) is sucked and discharged by reciprocal motion of the piston, and thereby it acts as a skew piston pump for moving the fluid in one direction by way of cylinder holes. For example, in Japanese Patent Application of Laid-Open No. H06-10825, a skew axial piston pump and motor is disclosed, in which the working fluid sucked into the cylinder holes from the low pressure side of a distributor opposing to the end face of the cylinder block is discharged to the high pressure side by rotation of the cylinder block.
The cylinder block like the above is generally formed of ferriferous metal material in order to have enough mechanical strength, and airtightness is required at the same time so as to retain the fluid pressure in the cylinder holes. Accordingly, if it is manufactured by a method of powder metallurgy, it is necessary to prevent leak of fluid by sealing the pores in the sintered material. For this purpose, hitherto, the ferriferous sintered body is treated by steam process (see, for example, Japanese Patent Publication No. 57-026322 and Japanese Patent Applications of Laid-Open Nos. S56-169705 and H05-078875), and by the action of heated steam, the surface of sintered body or the inner wall of pores opened to the surface is oxidized to form a layer of triiron tetraoxide (Fe3O4), and open pores existing in the surface portion of sintered body are sealed by utilizing volume expansion due to the oxidation of iron.
The cylinder block manufactured by steam treatment of ferriferous sintered body obtained in accordance with powder metallurgy is wear resistant, since a rigid iron oxide layer is formed on the surface. However, the opposite members such as piston and distributor for supplying fluid are easily worn on the contrary. In particular, the cylinder block end face of the cylinder hole bottom side rotating and sliding in a state pressed to the distributor is demanded to enhance the sliding property so as not to damage the opposite members, in order to extend the service life of the working system.